Regenerative Step Pool Storm

Conveyance (SPSC) – also known as Coastal Plain Outfalls

Design Guidelines

Draft Revision 3 Under Review Not for Distribution

Home Port Farms - Immediately after i

Home Port Farms - One year after construction

Homeport Farms - Six years after construction

Original : June 2009 Revision 1: August 2010 Revision 2: November 2010

Ron Bowen, P.E.

Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Technical Advisory Committee

This document was prepared by Hala Flores, P.E., with input from Dennis Mcmonigle and Keith Underwood. This document is maintained on the Anne Arundel County website and can be accessed through http://www.aacounty.org/DPW/Watershed/StepPoolStormConveyance.cfm.

Updates and revisions to this document are reviewed by the Technical Advisory Committee as follows.

Ronald E. Bowen, P.E., DPW Director Christopher J. Phipps, P.E., DPW, Chief Bureau of Engineering Kenneth Fleming, P.E., DPW, Assistant Chief Bureau of Engineering Ginger Ellis, DPW, Watershed Ecosystem, and Restoration Division Chief Keith Underwod, Underwood and Associates Hala Flores, P.E., DPW, Watershed Assessment and Planning Program Manager Janis Markusic, DPW, NPDES-MS4 Coordination/Ecosystem Assessment Program Manager Dennis McMonigle, DPW, Environmental Restoration Project Manager Christopher Soldano, OPZ, Deputy Director Richard Olsen, DPW, Infrastructure Management Division Elizabeth Burton, OPZ, Chief Engineer George Eberle, I&P, Assistant Director John Peacock, I&P, Code Enforcement Administrator Jim Stein, Vernon Murray, Chris Maex, and Charles Henney, AACO SCD Earl Reaves, I&P, County Forester

Summary of Revisions:

August 2010 Revision 1: (a) Added language to clarify when an SPSC system can and cannot be used as part of the ESD treatment train.

November 2010 Revision 2: (a) Replaced the d50 cobble size definition with the d0 definition indicating that the cobble design size is the minimum allowable cobble size to be used rather than median stone size. (b) Added a clarification on the minimum and maximum allowable length of pools and riffles (c) Added a design checklist. (d) Added an inspection checklist. (e) Added guidance pertaining to sequence of construction and erosion and sediment control measures.

April 2011 Revision 3: The MDE Stormwater guidelines prescribe that filtering systems shall fully drain within 72 hours, however unlike infiltration systems, no groundwater separation is specified for filtering system. Due to this, revision 3 eliminates the requirement for groundwater separation from the filtering system. However, to ensure that the filtering mechanism works as designed, the design water quality filter ponding depth in the pools, also known as seepage head, shall be available for storage during storm events and not inundated by seasonal ground water. Further, the pools shall drain down within 72 hours to their design levels.

April 2011 Revision 3: Added horizontal and vertical setback requirements for SPSC systems. Added “Regenerative” to the practice name for consistency with EPA TMDL/WIP publications.

April 2011 Revision 3: Width/Depth ratio for Riffle/Weir section shall not be less than 10W:1D. Further, pool depths shall not exceed 3 ft.

Page 2 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Important Note:

This document features design guidelines and procedural steps to aid design engineers in sizing a Step Pool Storm Conveyance (SPSC) system. It is the responsibility of the design engineer to check the feasibility and acceptability for using these systems at their project site. SPSC can be used in lieu of stormdrains as roadside conveyance/attenuation systems. SPSC maybe used for peak flow management or steep slope stability treatment and are considered structural BMPs if they are sized to accommodate the State of Maryland volume control requirements specified in Chapter 2 of the State manual. In general, SPSC may be used as a structural stormwater management device to provide water quality treatment as part of the treatment train or at the downstream outfall after all Environmental Site Design (ESD) techniques have been exhausted to the Maximum Extent Practical (MEP) as dictated in the State of Maryland SWM manual. Under special circumstance, the SPSC may be used as part of the ESD when the design conforms to the criteria found in Chapter 5 (State of Maryland SWM Manual) for microbioretention or bio- swale and the general configuration conforms to the principles of ESD: using small-scale practices distributed uniformly around the site to capture runoff close to the source. While SPSC systems can be implemented on steep slopes, in no circumstance can water quality credit be claimed for SPSC segments with a longitudinal profile slope that exceeds 5 percent. Additionally for storm conveyance segments that exceed 5 % in longitudinal slope, it is imperative that a registered geotechnical engineer be enlisted to check the proposed design and its adequacy for use as a steep slope treatment measure.

Introduction:

SPSC are open-channel conveyance structures that convert, through attenuation ponds and a sand seepage filter, surface storm flow to shallow groundwater flow. These systems safely convey, attenuate, and treat the quality of storm flow. These structures utilize a series of constructed shallow aquatic pools, riffle grade control, native vegetation, and an underlying sand/woodchip mix filter bed media. The physical characteristics of the SPSC channel are best characterized by the Rosgen A or B stream classification types, where “bedform occurs as a step/pool, cascading channel which often stores large amounts of sediment in the pools associated with debris dams” (Rosgen, 1996). The pretreatment, recharge, and water quality sizing criteria presented in these guidelines follow closely the State of Maryland’s criteria for a typical stormwater filtering device. These structures feature surface/subsurface runoff storage seams and an energy dissipation design that is aimed at attenuating the flow to a desired level through energy and hydraulic power equivalency principles.

SPSC structures can be designed to provide energy dissipation and extreme flood conveyance/attenuation functions, as well as recharge and water quality treatment in excess of ESD. The inherent energy dissipation achieved in the step pool design is directly linked through hydraulic design computations to reduced stream power and bank shear stresses in the receiving streams, thus satisfying more directly and effectively the State of Maryland’s channel protection requirement and additional local requirements for flood attenuation. The reduced energy and

Page 3 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

velocity at the downstream end of these structures result in reduced channel erosion impacts commonly seen between conventional stormwater practice outfalls and ultimate receiving waters.

SPSC structures are generally best suited in natural ravines and are the preferred method of storm water conveyance throughout the water train. ESD techniques such as alternative pavement, greenroofs, rooftop disconnections, vegetated swales, etc., should be considered and utilized to the MEP in the upstream area of a proposed SPSC system.

A secondary benefit provided by the pools and plant material is to reduce flow velocity and enhance the removal of suspended particles and their associated nutrients and/or pollutants. Additionally, uptake of dissolved nutrients and adsorption of oils and greases by the plant material yield secondary water quality benefits above and beyond the benefits achieved through the primary water quality sand/woodchip mix filter.

The design material and plant list featured within this document has been adapted to the Anne Arundel County coastal plain environment. The materials used within the SPSC, to the extent possible, are taken from the coastal plain. The sand media is quarried throughout the region and can be readily obtained. The boulders found in these systems are sandstone (e.g., bog iron, iron stone, ferracrete). Sandstone’s porosity, as well as its ability to retain water, allows it to naturalize quickly, providing habitat for ferns, moss, and other organisms that persist in these systems. While sandstone is the preferred material for use as boulders within these systems, granite may be substituted if sandstone availability is demonstrated to be of a concern. The use of other alternative boulder material must be approved by the Anne Arundel County reviewer and/or project manager. Under no circumstance is limestone or concrete permitted in the construction of SPSC systems.

Construction specifications for the SPSC can be found in the Anne Arundel County Design Manual, Standard Specifications and Details for Construction document.

General Design Situations

SPSC structures consist of an open channel conveyance with alternating riffles and pools. These systems are best suited for ditches, outfalls, ephemeral and intermittent channels with longitudinal profile slopes that are less than 10%. However, the design can be easily adapted for sites where the slope exceeds 10 percent. For these sites, the size and quantity of the cobbles and rows of boulders inherent in the design computations are increased to mitigate for the stability issues associated with steep slopes. It is noted that the utilization of two or more rows of boulders typically will result in a water cascade. In extreme slope situations (>50%), the designer may elect to use impricated riprap, gabion baskets, retaining walls, drop stormdrain structures or other structural/geotechnical slope treatment methods to safely traverse the grade.

In order to preserve the integrity and habitat functions of non-tidal wetlands and streams, the designer is encouraged to minimize to the extent possible changes to the drainage pattern. This is achieved by placing proposed SPSC systems within the site following the native drainage paths. While this may result in temporary construction impacts, in the long run it will preserve

Page 4 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

the hydraulic input which is crucial to the survivability of habitat functions within non tidal streams and wetlands. It should be noted though that the computations presented in this document do not address water budget or sediment transport/competency issues that are generally encountered in perennial streams and wetlands. Due to this, it is imperative that designers additionally check these issues for SPSC systems proposed within perennial streams and/or wetlands.

"The current condition of single gravel-bedded channels with high, fine grained banks and relatively dry valley-flat surfaces disconnected from groundwater is in stark contrast to the pre- settlement condition of swampy meadows (shrub-scrub) and shallow anabranching streams." (Walter, R., & Merritts, D. 2008). Current stormwater management regulations require that proposed development plans include appropriate mitigation measures and be contingent on the presence of a stable outfall. According to the Anne Arundel County Watershed Master Plans, problem area inventories such as erosion, buffer deficiencies, headcuts, infrastructure impacts, and suboptimal habitats are notable in varying degrees in more than 90 percent of the surveyed stream segments. For projects that drain to stream channels with active incisions, it is imperative that proper tie-in design be established between the SPSC system and the connecting downstream channel. This could be accomplished by installing an in-stream weir at the proper elevation to promote upstream flooplain connection and prevent headcut erosion from unraveling the proposed SPSC systems. An example design solution for tying the SPSC systems with downstream incised perennial channel is presented in this document. It is noted that each case should be evaluated carefully and that design engineers propose appropriate solutions based on the individual circumstance surrounding each case. Additionally, the designer engineer is responsible for notifying and obtaining all required approvals from the Local, State and Federal authorities.

It is important to acknowledge that each site has unique and defining features that require site- specific design and analysis. The guidance provided below is intended to provide the fundamentals for sizing the facility to meet the regulatory requirements but is not intended to substitute engineering judgment regarding the validity and feasibility associated with site- specific implementation. Designers need to be familiar with the hydrologic and hydraulic engineering principles that are the foundation of the design and they should also enlist the expertise of qualified individuals in stormwater management and stream restoration plantings with respect to developing appropriate planting plans and habitat improvement features.

Hydraulic Design of SPSC Systems

SPSC systems can be used to achieve a zero surface water discharge. This is accomplished by converting the 100-year surface discharge to subsurface flow/spring head seep. The design of the SPSC should be based on specific established restoration goals for the project. The sand/woodchip mix filter media is specifically required for retrofit projects with water quality restoration goals. The depth and quantity of the pool structures is linked to water quality, energy dissipation, and flow attenuation/peak management requirements. Additionally the SPSC design parameters maybe determined based on the specific needs to retrofit an existing eroded channel outfall for instance. The dimensions of the riffle and pool segments are designed in a manner to ensure adequate and safe conveyance of the design flow. The downstream tie-in to the receiving

Page 5 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

channel aims to correct existing downstream deficiency, such as incision and erosion, and promote long-term stable outfall conditions. This is a requirement for all proposed developments. The downstream tie-in design may result in additional water quality benefit for the contributory drainage area, however, this may not be claimed as water quality mitigation for new development related impacts, rather this benefit may be claimed for select redevelopment projects and will be evaluated by the Anne Arundel County Department of Public Works for consideration as credits toward the County’s National Pollution Discharge Elimination System (NPDES-MS4) permit conditions. The construction cost of these systems makes it imperative for the design engineer to carefully target the specific restoration goals prior to providing a design solution. The following steps have been formulated to aid the designer engineer in preparing the minimum design elements for the SPSC.

1. Develop the hydrologic design parameters for the project • The drainage area should be delineated to the outfall point of the SPSC and the connecting channel tie-in location if applicable. In new development projects, ESD shall be used to the MEP such as to minimize alterations to the existing drainage patterns for the site. • Using TR55, determine the flow path, time of concentration, and weighted runoff curve numbers for all points of investigations and required landuse scenarios. • Using TR20, determine the 1, 10, and 100 year peak discharges for all points of investigations and required landuse scenarios. • Include pertinent hydrology parameters and results pertaining to all points of investigations and required landuse scenarios on the construction plans.

2. Establish and quantify the restoration goals for the project • Establish the goals for the SPSC. The goals may include, but are not limited, to the following:

□ Providing safe open channel surface conveyance in lieu of stormdrains. □ Providing structural water quality mitigation in excess of ESD to the MEP. □ Providing slope and outfall stabilization □ Subwatershed retrofit to meet specific NPDES and/or TMDL requirements.

• Quantify the required pretreatment, recharge, and water quality volumes for the proposed development. The required volumes should be computed per the Anne Arundel County’s current stormwater management regulations.

• The restoration goal for the project and the provided quantities of water quality treatment shall be listed on the construction plans.

3. Map the horizontal alignment for the project • Develop a geometric plan sheet showing the SPSC alignment with stations and tabulated coordinates. The SPSC will be placed in the landscape following a

Page 6 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

curvilinear flow path whenever possible that generally follows the shape of the ravine or localized drainage path. • Special attention should be followed to minimize impacts to natural features. This could be accomplished through innovative/adaptive construction phasing and tree protection plans. • Special effort shall be made by the designer to avoid entrenched linear designs of the step pool structure. Storage opportunities on the floodplain lateral to the structure should be utilized to the maximum extent possible. • Measure the length of the reach along the plan view alignment from its input to the discharge location. This length shall be described in the design formulas as Ldesign. The discharge location shall be at the receiving channel. In the event that the receiving channel is incised/disconnected from the floodplain, an in-stream weir may be utilized to connect the receiving channel with the floodplain. A horizontal alignment shall be established for the in-stream work. Design guidelines for the in-stream weir tie-in are included in this document.

1+50

0+50 1+00

1+50

Ldesign 0+00 2+00 1+00

In-stream SPSC Flow Flow 0+50

0+00

Page 7 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

4. Map a preliminary vertical alignment for the project

• Measure the elevation difference ”ΔE” between the top and the bottom of the proposed SPSC. In the event that the proposed SPSC connects to an incised downstream channel, the elevation of the floodplain terrace shall be used as the downstream elevation. An in-stream weir design with a top of weir elevation set at the floodplain terrace is required at the tie-in location. • Compute the average outfall slope, S, by dividing ΔE by Ldesign. • SPSC segments utilized for water quality shall not exceed 5% in longitudinal slope. If the overall slope exceeds 5%, estimate the length of boulder cascade to use for traversing the grade. Boulder cascades maybe placed at 2H:1V or 50% slope. A maximum 5 ft of vertical drop shall be permitted at any single cascade location. Multiple cascades maybe required along the length of the project to traverse steeper grades. The location of the cascade shall be selected to minimize site disturbances and environmental impacts. • To simplify the calculations, assume that the length of the pools is equal to the length of the riffles at 10ft. The actual minimum length of pools shall not be less than 10 ft. Cobble riffle segments may not exceed 10 ft in length so as not to build excessive energy and maybe substituted for sandstone weirs when the flow is excessive. • Assume a fixed one foot drop along the length of the riffle. • Assume a minimum 18 inch fixed pool depth. • Assume no elevation drop along the length of the pool to allow for dead storage ponding and promote filtration/infiltration. • Alternate pool and riffle channels using an even length distribution along the horizontal alignment. Three consecutive pools separated by cobble riffle grade control structures shall be used following a cascade. • Using assumptions above, ΔE-ΔEcascade in feet will equal the number of riffle channels and associated pools. • Boulders shall be used in-stream to transition the in-stream weir with the downstream bed elevation. A maximum 5% longitudinal profile slope shall be used to establish the grade transition.

L Riffle L pool

(10 ft max.) (10 ft min.) Silica Cobbles Boulders In-stream Boulders hf Typical (18 in. min.) In-stream 100 – Year floodplain 1 ft (typ.) may inundate last SPSC structure

Sand/Wood Chip Mix

df (riffle) Existing Ground Filter Fabric Footer boulder shall extend 6 inches below the lowest point in the excavated pool df (pool) Typical Profile – Alternating Pools and Riffles min 18 inches Sand Mix “Sandbags from E&S phase maybe left in place” Page 8 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Cascade Pool #1 Pool #2 Pool #3 Max 10ft @ 50% Slope

Boulders Silica Cobbles hf cascade hf (Typical) (per design) h (Typical) f

Filter Fabric

Cascade Boulders shall Sand/Wood Chip be double lined Mix

Existing Ground Cascade Profile – Three Pools following Cascade

5. Design the typical cross-section for the riffle/weir/cascade and pool channel segments • The riffle/weir/cascade and pool channels shall be parabolic in shape. • Design the riffle/weir/cascade and pool channels to carry the Qdesign for the unmanaged 100-year storm flow in a parabolic shape. The area and hydraulic radius of a parabola are computed as follows: 2WD Area = Mathematical Solution 3

2W 2D Hydraulic Radius = Chow,1959 3W 2 + 8D2 W (8 ft min.)

D

Riffle Section through Boulder

2 x d0

Riffle Section through Cobble

• The minimum freeboard for lined waterways or outlets shall be 0.25 ft above design high water in areas where erosion-resistant vegetation cannot be grown and maintained. No freeboard is required if vegetation can be grown and maintained. (USDA, 2006.)

Page 9 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

• Select a trial constructed riffle channel width (W). The width is the dimension perpendicular to the flow. • Select a trial constructed riffle channel depth (D). The Width/Depth ratio shall not be less than 10. • The dead storage depth within the pool shall not be considered when checking for adequacy of conveyance • Design using a trial cobble with a d0 of 6 inches. The calculated d0 shall be the minimum stone size diameter to be used in riffle channels. The density of the stone shall be specified. The depth of the cobble material is equal to 2 x d0 (MDSHA, Highway Drainage Manual, 1981). Boulders shall be used to line cascade segments. • Calculate the Manning’s n roughness coefficient based on the constructed depth, D, associated with the 100-year ultimate flow conditions and the cobble size:

1/6 n = D / (21.6 log (D/d0)+14), (USDA, 2006). Where: n = Manning’s n, use 0.05 for cascades. D = depth of water in the riffle channel associated with unmanaged 100-year Q design, ft., d0 = Minimum cobble size, ft

• Use the Manning formula to calculate the flow and velocity associated with the trial parameters D, W, and d0. The design flow shall meet or exceed the 100-year ultimate flow conditions.

2/3 1/2 Q = (1.49/n) (A) (Rh) (S)

Where: Q = 100 year ultimate flow (cfs) 1.49 = conversion factor n = Manning’s n, determined by USDA, 2006 equation A = cross-section area of a riffle channel, which for a parabola = 2/3(W)(D), where W is top constructed width (ft) and D is the constructed depth (ft) Rh = hydraulic radius (ft), calculated using Chow 1959 relationship for parabolas S = average slope over entire length of project (ft/ft) V = velocity in the riffle channel (ft/sec), V = Q/A

• Using small incremental depths (0.1 ft), develop a hydraulic rating curve/table for the channel to ensure that subcritical flow conditions prevail to the greatest extent possible. This is achieved by calculating the Froude Number, Fr. A Froude number exceeding 1 indicates that the flow is supercritical. A Froude number of less than 1 indicates that the flow is subcritical in nature. The Isbash coefficient for high turbulence should be used when sizing the cobble stones to accommodate supercritical conditions. Increasing the cobble size or the width depth ratio of the riffle channel can increase roughness and reduce velocity. This can further assist in meeting subcritical flow conditions. Refer to the design example at the end of this document for an example of a hydraulic rating curve table.

Page 10 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

V Fr = gD

• The design velocity shall be checked to ensure that it is below the maximum allowable velocity estimated from the Isbash formula below (NRCS, 2007). A graphical solution of the Isbash formula is also shown. This will be an iterative design process. Spreadsheets can be used to streamline the calculations.

0.5 ⎛ γ − γ ⎞ ⎜ s w ⎟ 0.5 Maximum Allowable Velocity = C × ⎜2× g × ⎟ × ()D50 Isbash Formula ⎝ γ w ⎠

Where: C = 0.86 for prevailing supercritical flow and 1.2 for prevailing subcritical flow g = 32.2 ft/sec2

γs = stone density (lb/ft3) γw = water density (lb/ft3) d0 = cobble stone diameter (ft)

Graphical Solution for Isbash technique Figure TS14C-6, (210-VI-NEH, August 2007, TS14C-4

Page 11 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Cascade Ineffective 5 ft elevation Flow Areas drop (max.) Followed by 3 consecutive A A’ pools

Flow B B’

Riffle – Pool Sequence (Typical) Cascade Sequence

W (8 ft min.)

D 2 x d0

Df (18 in min.) Sand/Woodchip Mix df (riffle)

Wsand (2 ft min.) Section A-A’ Riffle Weir Cross Section through Cobble

hf (18 inch min.)

Sand/Woodchip Mix df (pool) 18 inches min.

Section B-B’ Pool Cross Section

Page 12 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

• The constructed depth of the typical pools (hf) and the pool directly following a cascade (hf cascade) shall not be less than 18 inches and shall not exceed 3 ft. Floodplain storage should be sought in the event that additional volume of storage is required. This will result in a pool geometric design with less than 3 ft of embankment and will meet the exemption criteria for the Soil Conservation District small pond approval. The minimum design depth of the pools shall be estimated based on the use of the solved form of the Bernoulli conservation of energy equation shown below. The Bernoulli equation was solved to achieve a pool channel velocity of 4 ft/sec. D and V correspond to the riffle/cascade channel design depth and velocity respectively.

V 2 h or h = D + − 0.25 f f cascade 2g

• To ensure stability, the pools shall be constructed with a minimum side slope of 3H:1V.

• The sand/wood chip filter media shall meet the MDE ESD specifications for sand. Sand shall conform to AASHTO-M-6 or ASTM-C-33, 0.02 inches to 0.04 inches in size. Sand substitutions such as Diabase and Graystone (AASHTO) #10 are not acceptable. No calcium carbonated or dolomitic sand substitutions are acceptable. No “rock dust” can be used for sand. The woodchips are added to the sand mix, approximately 20% by volume, to increase the organic content and promote plant growth and sustainability.

• The minimum depth of the sand/woodchip mix filter media, df, below the invert of the pools, shall be 18 inches.

• Filter fabric shall be placed under all boulders. Refer to design figures for placement location. To prevent undercutting, a continuous sheet of filter fabric shall be used along the cross-section. Filter fabric shall not be placed in the pools so as not to impede filtration.

• The sandstone boulders serve as the weir component of the riffle grade control structure. The boulders should be arranged in a curved manner as shown on the riffle pool sequence schematic and the sandstone weir elevation view. This arrangement is intended to encourage flow deflection to the center of the pool and the creation of ineffective flow areas near the channel banks. To achieve this, the sandstone boulders shall be arranged horizontally in the center of the channel and the arms on either side of the channel shall be extended parabolically/ approximately 20 degree angle longitudinally to the center of the pool. The sandstone boulders should be

Page 13 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

sized by the engineer to be at least 3 to 4 times heavier than the riffle channel cobble. Typically, the diameter of sandstone boulders shall not be less than 2 ft. in length. The typical boulder size shall be designed and specified on the plans by the engineer to best fit the channel shape. i.e., smaller cross-sections will require smaller boulders, while larger channel cross-sections may require larger boulders. The sandstone boulders should be tabular in shape to allow for maximum interlocking.

• The footer rocks provide added stability to the sandstone boulder in the event that excessive erosion is experienced in the energy dissipation pools. The footer rocks may not be necessary in the event that the utilized sandstone boulders size is adequately anchored (six inches below the lowest elevation point in the pool). The footer rocks shall be equivalent in size to the sandstone boulders and should be tabular in shape to allow for maximum interlocking. Boulders shall be stacked as a double layer when used to armor a cascade. All footer rocks shall be anchored 6 inches below the lowest excavated elevation in the pool.

6. Design the in-stream weir tie-in structure (If applicable) • The in-stream weir shall be set approximately 30 ft downstream of the tie- in location. The top elevation of the weir shall be set at the desired/historic flooplain elevation as determined appropriate by the engineer and approval authority. This is intended such as to impede headcut through the SPSC and inundate the floodplain for all flows above the base-flow conditions, thus enhance the water quality conditions. To avoid the creation of a high hazard dam condition, the maximum height of the weir shall not exceed 3 ft. Multiple weirs, not to exceed 3 ft in height, maybe required upstream to traverse the grade gradually and connect incised channels to the floodplain. • The in-stream weir shall be connected longitudinally to the downstream existing grade through a maximum 5% slope boulder channel. This will ensure that flow velocities do not impede the fish passage. • Sand shall be used for filling the stream bed to the desired elevation. Sand bags utilized as part of the erosion and sediment control plan for creating instream diversion maybe left in place. Geotextile shall be used to separate the sand fill and the overlay boulders that line the channel. The boulders shall extend in cross-section to the 100-year floodplain elevation. • The in-stream boulders shall be sized such as to remain in place under the 100-year velocity and shear stress, and shall be placed in a manner such as to create maximum hydraulic friction. • The last one or two structures within the SPSC system may be inundated by the in-stream 100-year flood elevations. • HEC-RAS shall be used to estimate the in-stream 100-year water surface elevation. The HEC-RAS sections shall be extended upstream to the point where the existing and proposed 100-year floodplains converge.

Page 14 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

The proposed in-stream design shall not result in additional flooding “above and beyond the historic FEMA floodplain” within private property or degradation to the hydraulic adequacy of upstream public facilities.

In-stream Flow C

Max. 3 ft.

Sand Fill “Sandbags from stream diversion maybe left in place” C Existing Ground

Silica Cobbles In-stream longitudinal profile at SPSC tie-in location

In-stream Top of Weir In-stream Boulders In-stream 100-year floodplain

Existing Geotextile Ground

Sand Fill Sand/Wood Chip Sandbags from stream diversion maybe left Mix Max. 3 ft. in place

Most downstream SPSC riffle/pool Cross-Section of Connecting Stream structure at Weir Section C-C - Instream Cross-section at Weir

7. Check and adjust the design parameters based on the project goals • After implementing ESD to the MEP, determine if the remaining Water Quality Treatment Volume (WQv) is met by comparing the provided sand/woodchip mix filter bed area, A provided to the required filter bed area. The provided sand/woodchip mix filter bed area can be computed by multiplying the average width of the sand/woodchip mix media, where the provided sand/woodchip mix depth is at least 18 inches (MDE, 2000), by the length of the sand/woodship mix media, Lsand, in the direction of the flow.

WQ v x d f A f = , Filtering Sizing Criteria MDE 2000 K (h f + d f ) t f

Page 15 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Where, 2 Af = required sand/woodchip mix filter bed area (ft ) 2 A provided = provided sand/woodchip mix filter bed area (ft ) Wsand = width of sand/woodchip mix filter bed (ft), minimum = 4 ft. Lsand = length (ft) along the project (Lpre) 3 WQv = prescribed Water Quality Volume (ft ) df = sand/woodchip mix filter bed depth (ft), use minimum 24 inches (Average of df (pool) and df (riffle). K = coefficient of permeability of filter media (ft/day), use K = 3.5 for sand/woodchip mix. hf = Depth of Pool (ft), minimum 18 inches. tf = design filter bed drain time (days), use 1.67 days as recommended by MDE for sand mix filters.

• If the required WQv and Rev are not met by the proposed SPSC design, then increase the filtration capacity by adjusting the depth of pools, width of sand/woodchip mix filter, or length of the facility. If these adjustments are not feasible, utilize other BMP measures to provide the remaining water quality volume.

• In situations where the existing soil, underlying the proposed SPSC, is confirmed through “borings” to be highly infiltratable, the designer may utilize the MDE water quality sizing criteria for an infiltration basin in lieu of filtration. This is prescribed so the designer engineer is not forced, under certain circumstances, to replace highly infiltratable native soil with non-native filter bed material. In order to claim water quality credit, the sand/woodchip mix shall have sufficient separation from the groundwater elevation as prescribed by MDE SWM manual for filtering systems . design ponding depth/head, hf, intended to drive the seepage through the filter shall be entirely above the seasonal high groundwater elevation.

• The proposed SPSC will satisfy peak management flow requirements if two conditions are met:

□ First, adequate storage volume within the pools and sand/woodchip voids shall be provided to meet the required storage volume/quantity management for the project □ Second, it must be demonstrated that the design renders the hydraulic power equivalent to the predevelopment/desired hydraulic power through the proposed energy dissipation pools.

• To achieve the conditions above, the designer must compare the required peak management storage volume with the combined volume within the pools and the volume in the voids within the sand/woodchip mix. A 30% porosity shall be used for the sand/woodchip mix to calculate the volume within the voids. The total provided storage shall exceed the required storage volume for the design peak management storm. Second, the

Page 16 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

selected design for the SPSC must be checked using the conservation of energy principles to ensure that the hydraulic power is adequately reduced to design/predevelopment levels. This is achieved by equating the predevelopment or reference condition hydraulic power to the post development hydraulic power and solving for the equivalent added stream length/volume of storage needed to render this power to the desired condition. The conservation of energy principles are then utilized to convert the energy loss within this horizontal length to an equivalent vertical drop. The vertical drop is then converted to multiple drops that are distributed along the system in a manner that result in the least site disturbances. The provided quantity/volume of pools is then compared with the calculated quantity/volume of pools. If the provided pool storage is less than the computed/required pool storage, then additional SPSC design measures or additional upland management strategies must be taken to reduce the inflow and in turn the hydraulic power. Refer to the figure below for a demonstration of the SPSC-provided volume of storage and input parameters for the conservation of energy computations. It should be noted that equating the geometric configuration of a multiple pool system to one pool with an area equal to the cumulative areas within the individual pools is a conservative measure and is used to simplify the hydraulic power routing computations. It is expected that cumulative roughness and headloss within the multiple pool configuration to be much higher than the individual pool configuration.

Storage Volume in Pools Vin = Qpost

Di

D out Dout

Driffl Dout Dpoo L Storage Volume in Voids Design Water Surface Df x L x Wsand x Porosity Sand/Woodchip mix ~ Porosity = 30% ~ Df (Average filter bed area= (Driifle +Dpool)/2 Vin = Qpost

Di Vout = Qpost

Dout = Dout1 + Dout2 + Dout3 Driffl

Dpoo

Page 17 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

• The following steps should be followed in checking the before/after hydraulic power:

¾ Compute the predevelopment/design and post development hydraulic powers by substituting the predevelopment and post development discharges in terms of Q in the hydraulic power equation. The hydraulic power is expressed in the units of lb/sec. Hydraulic Power = γ x Q x S, where γ is the unit weight of water = 62.4 lb/ft3 Q corresponds to the MDE 2000 CPV discharge S is the slope of the outfall channel in percent

¾ Equate the predevelopment/design and post development hydraulic powers and solve for the needed added stream length. ¾ γ x Qpre x (ΔE/Lpre)= γ x Qpost x (ΔE/Lpost)

¾ The elevation difference between the top and bottom of the project and the unit weight of water will remain constant, therefore, the channel protection requirement could be expressed in terms of a required additional stream channel length Ladd, needed to render the post development hydraulic power equivalent to the predevelopment hydraulic power. • Ladd = Lpre x (QPost/QPre) - Lpre ¾ The required headloss due to friction through the Step Pool Storm Conveyance system can be calculated using the Darcy- Weisbach equation. By substituting Ladd for L, this headloss becomes equivalent to the energy loss within an added stream channel of length Ladd. The friction factor can be calculated using established relationships between Darcy-Weisbach friction factor and the Manning friction coefficient listed in Chow, 1959. The Darcy –Weisbach headloss equation is as follows:

2 fLaddVout Friction head loss = 2Dout g

¾ By substituting the required headloss term in the Bernoulli conservation of energy equation, the total combined design depth in ft of all proposed pools shall be at least equal to the “Dout” term embedded in the Bernoulli conservation of energy equation depicted below. If the total combined depth in ft of all proposed pools is less than the calculated “Dout” term, then additional pools are required or alternatively the pools could be made deeper. Solve for the “Dout” term using trial and error techniques or available commercial solver functions/calculators, (i.e.,

Page 18 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Microsoft Excel). The general and solved forms of the Bernoulli conservation of energy equation are shown below.

General Form of the Bernoulli Equation

(Potential + Kinetic + Static) Energies SPSC entrance = (Potential + Kinetic + Static) Energies SPSC outlet + Head loss within SPSCsystem

Solved form of the Bernoulli Equation

9Q2 9Q2 9 fL Q2 D + + ΔE = D + + add in 2 2 out 2 2 3 2 4gDin Win 4gDout Wout 4gDout Wout

Where, f = Darcy-Weisbach friction factor, the Chow 1959 equations below maybe used to relate the friction factor to a manning roughness: −1/ 3 2 f = 8gRh n Chow, 1959 http://www.water.tkk.fi/wr/kurssit/Yhd-12.121/www_book/runoff_6.htm

Ladd = additional channel length (ft) required to render the post development power to predevelopment conditions Vin = Design velocity at entrance riffle = V Din = Design depth at entrance riffle = D Win = Design width of riffle = W Wout = Width of the pool (ft) Vout = typical velocity of flow (ft/sec) in the pool, this term is unknown in the Bernoulli equation. Using flow continuity principals, this term could be expressed in terms of the CPV design discharge, Dout, and Wout. g = acceleration due to gravity = 32.174 ft/sec2 Dout = Solve for combined depth of flow in all pools (ft) and compare to the total provided pool depth

8. Finalize the cross-section and profile design for the project • Develop a grading plan based on the preliminary profile and cross-section typical design. • Adjust the preliminary profile dimensions to accommodate site specific concerns/impacts. Minimum design parameters for hydraulic, water quality, and quantity management criteria should be rechecked based on adjustments to the riffle/pool channels to ensure that safe and adequate conveyance is still maintained. • The sand/woodchip mix filter bed shall have a minimum depth of 18 inches under the riffle channel and a minimum width of 4 ft and shall be placed as the substrate drainage material along the entire project length. The actual dimensions of the sand/woodchip mix filter bed will be determined based on the required water quality volume.

Page 19 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

• Typically, construction of the SPSC system shall begin at the downstream end and proceed upstream to the project outfall. The outlet pool is designed to be placed at the lowest point in the project reach. This is often in the receiving wetland or stream/ floodplain, but can also be located in upland settings where the SPSC system discharges to another stormwater BMP or adequate storm conveyance system. • Footer boulders shall be placed at the interface of the pools and riffles as shown below. Additional boulders shall be placed on top of the footer bolders at the weir elevation upstream of the footer boulders to form the riffle channel parabolic shape. .

Riffle Pool

Weir Boulders (Parabolic Cross-Section)

Footer Boulders extend 6 in. below bottom of pool

Sand/ Wood Chip Mix

Existing Ground

• Continue the process of alternating pools and riffles/weirs up through the system to the entry pool. If the entry pool ties to an existing pipe outfall, additional armoring of the pool maybe needed to address the pipe exit velocities associated with supercritical hydraulic conditions. The designer may elect to use a larger size pool at the project entry to dissipate the outfall velocity and/or to address pretreatment concerns. • If the SPSC is proposed below a pipe system, it is desirable that the top invert of the weir associated with the entry pool is set at or above the invert of the discharge pipe or culvert. It is the responsibility of the design engineer to check the adequacy of the upstream drainage system • Course woodchips and compost should be used throughout the limit of disturbance for site stabilization. All areas should be hydro-seeded.

• It is advisable that excess materials, i.e., cobbles and boulders, be placed at the edge of the cross-section for use during the maintenance phase to correct any physical instability. • A direct maintenance access shall be provided to all sandstone weirs.

Page 20 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

9. Setback Requirements

• The minimum setback from the 100-year water surface elevation of the system to structures on slabs is 10 feet

• Systems located uphill of an existing house or structure shall be evaluated for possible adverse effects to the structure.

• The 100-year water surface elevation of a system located uphill of a building or structure that has a basement shall be no closer than 20 feet from the structure or the intersection of the structure foundation footing and the phreatic line associated with the overflow depth of the device, whichever is greater.

• The 100-year water surface elevation of a system located downhill of a building or structure that has a basement shall be no closer than 10 feet from the structure foundation or the intersection of the structure foundation footing and the phreatic line associated with the overflow depth of the device, whichever is greater.

• The 100-year water surface elevation of a system shall be located a minimum of 1 foot below the structure floor or basement floor.

• The 100-year water surface elevation of a system shall not be located within 25 feet of a retaining wall or the top of a slope that is 25% or greater. In no case shall the phreatic line associated with the overflow depth of the system intersect the existing or final ground surface of the retaining wall or slope.

• The 100-year water surface elevation of a system shall not be located within 50 feet of any residential development water supply well.

• The designer shall consider the proximity of existing sanitary septic drain fields when locating a new system. These systems can raise the localized groundwater elevation and therefore impact existing septic drain fields. The designer shall consult local Health Department regulations in these instances.

• The 100-year water surface elevation of a system shall not be located within 10 feet horizontally from any public sanitary sewer or house connection. Where the sewer pipe does not use "O" ring or glue weld schedule 40 connections, the minimum distance to the system shall be 50 feet horizontally.

Page 21 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

10. Sequence of Construction and Erosion and Sediment Control Notes • Under no circumstance can the SPSC system be used as a primary sediment control device during construction. Upstream controls and pump arounds are required during construction so as not to overwhelm the filtering capacity of the sand/woodchip mix. • The SPSC system shall not be finalized until all upstream construction is complete and erosion and sediment control measures have been removed. • Special attention shall be paid to the application of perimeter silt fence along the SPSC system so as not overwhelm the silt fence with concentrated flow and develop erosion within the SPSC floodplain and behind the stone structures. If erosion from sheet flow to the system is observed during construction, then a conveyance channel of adequate design should be implemented at the problem location.

11. Draft a planting plan

• The planting plan and proposed species must be reviewed and approved by the County project manager/reviewer prior to installation. Additionally, any plant substitutions must be approved by the project manager/reviewer before the substitute species are installed. • For projects within the airport zone, a sample list of MAA approved native plants is attached at the end of this document. A selection of approved trees with approved understory of shrubs and herbaceous materials should be provided. • Pay special attention to use of native material, diversity, and dense placement of plant material within appropriate wetness zones throughout the site (MDE, 2000). • Spray down a minimum 3 inch layer of compost throughout the site avoiding areas of ponding water. • Seed the entire site with Chewing Red Fescue. • Existing trees to be protected shall be marked clearly on the project plan view and planting plan. • The designer shall prescribe the use of course woody debris, ie. Inverted root wads, in the pool areas to enhance the soil porosity and create habitat for the biological community.

12. Prepare a monitoring plan and schedule of completion

• A monitoring plan must be prepared to address the specific restoration goals for the project. Structural stability and plants survivability are the two most pertinent components to monitor for private/developer built projects. These components shall be monitored for three years or as established in the plan review process. Enforcement of the monitoring

Page 22 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

conditions shall be tied to the asbuilt approval process and release of SWM bond. • The monitoring plan for SPSC shall include annual vegetation survey to document that planted species have 80% survivability and a biannual physical stability assessment. At the discretion of the project manager, annual benthic macro invertebrate monitoring using the Anne Arundel County approved protocols and storm event chemical monitoring for nutrients and sediments may be required. The monitoring plan shall also address all permit required project monitoring. • Routine/biannual maintenance of County-owned SPSC systems is prescribed for a period of three years. This includes, but is not limited to, mulching of devoid areas, diseased plant replacement and replanting if necessary, removal of excessive debris and invasive species. This is done to ensure plant survivability, and to monitor and ensure the structural integrity of the construction project by performing any routine structural maintenance necessary. • In the event that sediment accumulation exceeds six inches in the first year, the contractor shall spray down an additional layer of compost and replant the pool bottoms. • Unless encountered with natural groundwater perch, the filtering capacity shall be physically checked. If the filtering capacity diminishes substantially (e.g. water ponds in the pools the design ponding depth is not recovered afterfor more than 72 hours), the top few inches of discolored material within the pools shall be removed and shall be replaced with fresh material. The removed sediments should be disposed in an acceptable manner (e.g. landfill). • Direct vehicular maintenance access shall be provided to all sandstone weirs and pools. • A recorded maintenance agreement is required for all privately owned SPSC systems. • The operation and maintenance design detail and schedule shall be shown on the asbuilt plan. For privately owned structures, the maintenance agreement shall be officially recorded and the recordation number shall be included on the approved grading plans. • At a minimum, a maintenance plan shall include removal of exotic, invasive, and/or non-native plant species identified in the annual vegetation survey using methods approved by the County and by the Maryland Department of Agriculture.

Page 23 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

13. Design Checklist

SPSC Item Check Yes/No Reviewer Comments Delineate drainage area, landcovers, and soil to the most downstream point of the SPSC system. Develop TR55/TR20 model run to calculate the pre- development and post-development peak discharges. Hydrology Utilize MDE standards and TR-55 to calculate the required water quality volume and water quantity volume of storage to be controlled within the system. Conduct a downstream investigation to check the adequacy of the outfall system. Check the conveyance design (width, depth, slope) to ensure safe conveyance of the 100-year storm over the riffle/weir/cascade channels and that stable design dimensions for the cobbles and sandstone boulders are provided. Check the calculated minimum pool depth to ensure that sufficient pool depth is provided to dissipate the upstream energies properly. Hydraulics Check the post-development stream power for the 100-year storm to ensure that it is rendered equal to the pre- development stream power. (Note: this requires that sufficient SPSC length and number of pools be provided) Does the storage volume within the pools and voids meet the required quantity management storage volume prescribed for the project and calculated using the MDE standards and TR- 55. Does the alignment follow the natural drainage path and Alignment efforts are made to avoid impacts to natural resources such as trees and wetlands? Tree Have specimen trees been identified and a tree protection plan Protection been developed? Does the SPSC system extend downstream to a point where the outfall is considered stable? Downstream Has adequate downstream tie-in/transition been provided to tie-in address downstream instability and to ensure the outfall remains stable? Have the riffle segments been placed with a slope flatter than 5%? Have the pool segments been placed with a slope flatter than Longitudinal 1%? Slope Have cascades been placed at no more than 1H:1V slope with double-lined boulders, and the height of any single cascade does not exceed 5 ft? Do the side slopes for the pool (from all unarmored segments) exceed 3H:1V. Does the depth of the pool exceed the minimum calculated Pool Design depth based on the upstream velocities? The design of the riffle and weir shall be modified such as not to result in pool depth exceeding 3 ft. Does the length of the pool exceed the minimum required 10 ft and allow sufficient length to accommodate the 3H:1V slope on unarmored sides? Riffle Is the channel parabolic in shape?

Page 24 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

SPSC Item Check Yes/No Reviewer Comments Channel Does the width, depth, and slope meet the design requirement Design and allow safe conveyance of the 100-year storm? Are the d0 cobble size adequate for accommodating the 100- year velocities Note: d0 cobble size shall be specified on the plan, profile, and cross-section. The d0 is the minimum diameter size for the cobble stone. Smaller material shall be rejected by the inspector. Are the boulders forming the weir 3-4 times larger than the calculated D0? Are the footer boulders extended/anchored at least 6 inches Weir Design below the lowest point of the scour pool? Does the cross-section for the weir safely convey the 100-year stone? Is filter fabric placed under the sandstone boulders? Are the cascades armored with sandstone weir over filter fabric and the height does not exceed 5 ft at any given Cascade location? Design Are three pools provided following the cascade, with adequate weirs separating each pool structure and designed in a manner to safely convey the 100-year storm? Has the designer provided a typical detail sections for the riffle, stone weirs and pools where needed and actual cross- sections along the alignment at frequent intervals to reflect changes in the grading? Note: the cross-sections shall be developed Cross- based on the geometric alignment and shall show the station numbers, section existing grade, proposed grade, and sand mix/stone structure detail. Drawings Has the designer shown the 100-year storm water surface elevation on the typical and actual cross-sections? Has the designer provided a longitudinal profile along the Profile centerline of the alignment and shown invert and top Drawings elevations of all structures and the 100-year water surface elevation? Has proposed grading been provided, and is minimum/maximum dimensioning requirements met? Has the 100-year water surface elevation been plotted on the Plan Sheets plan? Is the 100-year water surface elevation sufficiently contained within easements and is below all habitable structures? Has adequate erosion and sediment control plan been E&S implemented upstream of the SPSC system? Has a permanent and direct maintenance access and public right of way been provided to all sandstone weirs and pools Maintenance all public facilities? Has a maintenance agreement been signed and recorded for private SPSC structures Has a monitoring/maintenance plan been developed for Monitoring County owned systems as prescribed in the design guidelines Plan and is clearly shown on the plan? Has mulching and hydro-seeding been prescribed for the entire system? Planting Has the designer paid special attention to the use of native material, diversity, and dense placement of plant material within appropriate wetness zones throughout the site?

Page 25 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

14. Inspection Checklist

SPSC Item Check Yes/No Inspector Comments Alignment Does the alignment of the system match the alignment specified on the plan? Length Has the contractor provided sufficient SPSC length to meet the minimum requirement on the plan? Elevation Does the elevation difference from structure to structure Difference and from top to bottom of system match the design plan and profile? Number of Does the number of weirs match the number specified on Weirs the plan and profile? Outfall Is the connecting outfall physically stable with no signs conditions and of erosion and is the structure properly tied in to the tie-in outfall as specified on the plans? Does the number of pools match the number specified on the plan and profile? Pools Does the depth in any given pool exceed the minimum required pool depth as shown on the design plans? Cascades shall not exceed 5 ft in vertical height at any given location. Cascades shall be parabolic in shape with adequate width Cascades and depth to accommodate safe conveyance of the 100- year storm as specified on the plan. Cascades shall be followed by three consecutive pools. Cascades shall be underlined by filter fabric. Are the weirs curvilinear in the direction of the flow? i.e, are the boulders placed in a manner similar to a cross- vane that would direct the flow to the center of the pool and away from the banks? Is the cross-section parabolic with adequate depth and Weir cross- width for safe 100-year conveyance as specified on the section plans? Are the sandstone boulders forming the weir 3 to 4 times larger than the d0 for the cobbles and underlined by filter fabric as specified on the plans? Are footer boulders provided and extend to a depth that is six inches below the lowest point in the pool? Does the minimum size of the cobbles exceed the d0 cobble size as specified on the plans? Cobbles Are the cobbles rounded? Are they silica/bank run gravel? Has the contractor placed the required volume of filter sand mix below the system? Filter sand mix Does the filter sand mix include 30% wood chips by volume? Filter fabric Has filter fabric been applied under all sandstone boulder structures and as required on the plans? Has all the upstream erosion and sediment control measures been removed? E&S Have adequate conveyance systems been provided to address all lateral drainage (Note: The presence of erosion lateral

Page 26 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

SPSC Item Check Yes/No Inspector Comments to the system suggests poor grading and the need for lateral conveyance systems to intercept the flow.) Has mulch/compost been applied on the entire site? Has the entire site been hydro-seeded with Chewing Red Fescue? Plantings Has the site been planted with adequate number of shrubs and trees as specified in the plan? (Note: Pay special attention to use of native material, diversity, and dense placement of plant material within appropriate wetness zones throughout the site.) Maintenance Has a direct vehicular maintenance access been provided Access to all the weirs? For private Has a maintenance agreement been prepared, signed, and structures recorded? For Public Structures: The following items must be verified before the structure as-built is accepted by DPW and the bond is released SPSC Item Check Yes/No Inspector Comments For water quality ephemeral systems, Does the pools Filtering drain to an acceptable level where the design ponding Capacity head for the filter is fully recovered in do the pools drain 72 hours after a rain event? Have the contractor removed sedimentation (exceeding 6 Sedimentation inches) from the pools? Has the structure been monitored for at least 3 years? Specifically physical stability and plant survivability. Monitoring Have annual monitoring reports been submitted to I&P and IMD and were they favorable? If not, were deficiencies addressed? Plant Have at least 80% of the planted shrubs and trees Survivability survived 3 years after installation? Physical Are all sandstone boulders in place with no sign of bank stability or bed erosion throughout the length of the project? Invasive Have all invasive plant species been removed from the Species system and the entire system re-seeded?

Page 27 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

References:

Rosgen, D., 1996, Applied River Morphology, Wildland Hydrology.

Chow, V.T. 1959. Open Channel Hydraulics, McGraw-Hill

Jarrett, A.R. 1994, Water Management, Pennsylvania State University

Maryland Department of Transportation, State highway Administration, December 1981, Highway Drainage manual, December 1981.

United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), (210-VI-NEH, August 2007), Stone Sizing Criteria,

Maryland Department of the Environment (MDE) and Center for Watershed Protection, Ellicot City, MD, 2000, 2000 Maryland Stormwater Design Manual, Volumes I & II.

United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), 2006, Lined waterway Standard Code 468

United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), 1997, Ponds-Planning, Design, Construction: Agricultural Handbook 590, page 12

United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), 1996, Engineering Field Handbook Part 650, Chapter 16, Appendix 16- A, Figure 16A-1

Walter, R., & Merritts, D. (2008). Natural Streams and the Legacy of Water-Powered Mills, Science, 319, 299-304.

Page 28 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Design Example

A Step Pool Storm Conveyance system is proposed as a retrofit solution for 25 acres of land with a contributory impervious cover of 10 acres. The TR20 peak discharges for the project are as follows:

Peak Discharge (cfs) – Tc = 20 minutes Upstream Landuse Scenario 1- year 2-year 10-year 100-year Desired/Reference Conditions 2 5 12 20 Ultimate Conditions 3 8 16 31

The outfall invert elevation is 100ft, and is connected to an incised stream under existing condition via 430 ft long eroded gully. The longitudinal slope for the connecting stream is 2%. The stream bed is incised by 6 ft below the historic floodplain terrace located at elevation 70 ft.

Calculate the SPSC geometric design parameters needed to safely convey the 100-year post development discharge, provide quantity management for the 10 year storm, and provide water quality treatment for the first inch of runoff. Estimate the outfall mitigation requirements for the project, if any.

Solution: A design spreadsheet/calculator was used to formulate the problem and solution. The length of project was specified as 430 ft. The elevation drop over the length of project was specified as 30 ft. Water quality credit may only be claimed for project segments with a longitudinal slope of less than 5%. Due to this, a 17 ft long - 50% slope boulder cascade will be utilized. This leaves 413 ft of SPSC at 5% slope. Assuming a 1-ft drop over the riffle segments and no elevation drop for the pool segments, then 17 pools and 17 riffle segments, at a design length of 12 ft will be needed. The riffle channel and cascade cross-sections were designed using Manning equation such as to safely convey 31 cfs associated with the ultimate condition for the 100 year storm. A trial width for the

riffle was selected (20 ft) and a cobble size with D0 = 8 inches. A trial depth was selected at 1.0 ft. This results in a riffle channel side slope of 10H:1V, which is acceptable. The spreadsheet/calculator was used to check the adequacy and stability of the selected geometric parameters. The selected geometry resulted in a channel that can accommodate 61.32 cfs. The velocity within the riffle channel is 4.6 ft/sec, the flow was subcritical. The cobble size and channel dimensions maybe reduced, however the conservative design is acceptable. A cascade cross-section with a 10 ft width and 0.5 ft depth will accommodate 33.62 cfs. Three followup pools with a minimum depth of 2 ft are required; however 3 ft pools are selected to meet the quantity management requirement for the project. See tabulated solution below.

Page 29 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

The filter bed area was sized using the MDE filtering criteria to treat the required 2,122 ft3 associated with an upstream drainage area of 25 acres with 10% imperviousness. An average 18 inch sand/woodchip filter media depth at 20ft width was selected. A 3ft ponding depth was selected. Utilizing the above parameters, the provided filter bed area exceeded the required filter bed area: see tabulated solution below. It should be noted that water quality credit was only claimed for 413 ft with a <5% longitudinal profile.

For this example, channel protection volume or peak flow management of the 1 year storm is required. The conservation of energy equation was used to calculate the minimum required pool depth that would result in a velocity of 4ft/sec or less within the pool. The minimum required pool depth was calculated at 1.5 ft/sec. However a 3ft deep pool was selected to satisfy the water quality requirement as shown above. This selection was used to preliminarily check the quantity management. This ensures that the velocity in the pool does not exceed 4 ft/sec. A total depth of 31.2 ft is required to render the hydraulic power associated with 100-year discharges = 31 cfs to hydraulic power levels associated with 100-year discharges = 20 cfs. However, the selected design provides additional energy dissipation with a total of 51.6 ft of total excavated pool depth.

A stable outfall is required at the SPSC tie-in location. A downstream field investigation of the connecting channel revealed an active incision/headcut with a height of 6 ft. Outfall mitigation is required. Two 3-ft high in-stream weirs are proposed at the SPSC tie-in location to arrest the downstream headcut and promote upstream channel aggradations to historic conditions. To ensure that the channel slope does not exceed the 5% maximum slope, a 60 ft boulder-lined transitional section will be required following each in-stream weir to traverse the longitudinal grade. A HEC-RAS model run is needed for the in-stream work to establish the proposed 100-year floodplain elevation and assess impacts, if any.

Page 30 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Checking the Channel Conveyance for the design flood Design Return Period (Yr) T 100 10 1

Time of Concentration in minutes (Before Development) tc 20.00

Pre development discharge (cfs) Qpre 20.0 12.0 20.0 Isbash curve for Stone Density = 165 lb/ft3 Cobble d50 Allowable Allowable Velocity size Velocity (Subcritical) (Supercritical)

Post development design discharge (cfs) Qpost 31.0 16.0 31.0 [inches] [ft/sec] [ft/sec] Cascade Design (maximum 5 ft Total available length (ft) L 430 drop per segment) Elevation drop over length (ft) delta E 30.0 Design Width (ft) 10.00 40.50.5

Total Cascade Length for Project (ft) Lcascade 17.00 Design Depth (ft) 0.50 45.17.1

Cascade Slope (ft/ft) Slopecascade 0.50 Roughness 0.05 55.78.0 Water Quality slope (ft/ft) Slope 0.05 A 3.33 66.38.7

Proposed Length of Riffle Channel (ft) Lriffle 12 θ 0.20 76.89.4

Number of Riffle Segments for Project Nriffle 17 P 10.07 87.210.1

Number of Cascade Segments for Project Npool 17 Rh 0.33 97.710.7

Minimum Required Length of Pool (ft) Lpool 12 Design Velocity (ft/sec) 10.09 10 8.1 11.3

Cobble d50 size (ft) - choose - 8 inches d0 0.67 Conveyed Q (cfs) 33.62 11 8.5 11.8 Cascade is adequate, use 2 12 8.8 12.3 Minimum top width of SPSC riffle channel (ft) W20.0 cascades Minimum Pool Depth 15 9.9 13.8 "Use 3 pools" following Minimum depth of SPSC riffle channel (ft) D1.0Cascade (ft) 1.80 18 10.8 15.1 hf, Minimum required dead storage depth within the pools of the SPSC (ft) hf 1.5 ok

Enter Desired Pool Depth (Maximum 4 ft) hf 3.0 subcritical/ok

Check Riffle Side Slope, Must be > 2H:1V 10.0 Adequate conveyance of design storm

Check the Froude Number to ensure Subcritical Flow Conditions 0.8

Selected Cobble Size is Adequate for Computed Roughness n0.06 100 year storm

Riffle Cross Section Area (ft2), for parabola A 13.33

Theta - Intermediate step for solving θ 0.20 Subcritical Flow is Predominant

Riffle Hydraulic Perimeter (ft), for parabola P 20.13

Riffle Hydraulic Radius (ft), using Chow 1959 Rh 0.66

Calculated Flow for design parameters (cfs) Q 61.32

Check Riffle Velocity (ft/sec) V4.60 Number of Pools 17

Page 31 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Provided total pool depth (ft) = 51.625

hydraulic power for return period 100 year storm is Checking Quantity Management sa tisfie d

USDA 2006, n expressed in terms of dout and d50 = 8 inches n0.02Required Volume of Storage (Rational Hydrograph)

The width at the entrance riffle Win 20.00 100 Yr 10 Yr 1 Yr

The velocity at the entrance riffle is calculated using

Manning formula calculator and Qpost for the 1 year storm Vin 5.16Required Volume of Storage (ft3) 59750 31284 7714

The depth at the entrance riffle is calculated using Manning formula calculator and Qpost for the 1 year storm Din 1.00 Volume provided in pools (ft3) 7436

Enter Trial Value : The total pool depth needed to render the power equivalent to 100-year predevelopment/desired levels. This should be compared against the total provided pool depth for adequacy. Dout 31.17 Volume provided in voids (ft3) 10320 8640 This is the typical top width of the dead storage pool parabolic areas, 10:1 side slope Wout 18 Provided Volume of Storage

The area is for a semi parabola Aout 374 (excludes infiltration) (ft3) 17756 Equivalent channel length (ft) required to satisfy the channel 9801 protection volume. Ladd 237 Infiltration Volume (ft3) Theta - Intermediate step for solving θ 1.43 Peak Management of 100 year storm is not satisfied

Hydraulic Perimeter (ft), for semi parabola Pout 66 Peak Management of 10 year storm is not satisfied

Hydraulic Radius, using Chow 1959 Rh 5.63 Peak Management of 1 year storm is satisfied

Darcy Weisbach friction factor expressed in terms of Ladd, f0.08

Solved using Solver equation: Bernoulli equation rewritten in terms of dout 0.00 Water quality requirement is satisfied in SPSC Checking Quality Management Site Drainage Area (Acres) A 25.00 Runoff Curve Number (Post Development) RCN 65.0 Contributory Impervious Area (Acres) I 10.00

Volumetric Runoff Coefficient Rv 0.41 Water Quality Volume, ft3 WQv 37208 val ue df (Av erage of Pool and Rif f le) 4.0

Width of sand filter (ft) Wsand 20

Length of sand filter, where slope < = 5% (ft) Lsand 413

Area of sand filter provided (ft2) Af Provided 8260 coefficient of permeability of filter media (ft/day) k3.50 height of water above filter bed- pool depth (ft) hf 3.00 val ue tf 1.67

Required filter bed area (ft2) Af Required 2122

Page 32 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Hydraulic Rating Curve for Riffle Channel – First Trial D 0 =8 inches – Adequate safe conveyance

D W A Rh n Q V Froude # 0.1 3.16 0.21 0.07 -0.17 -0.07 -0.33 -0.18 0.2 4.47 0.60 0.13 0.26 0.20 0.33 0.13 0.3 5.48 1.10 0.20 0.12 1.08 0.98 0.32 0.4 6.32 1.69 0.26 0.09 2.70 1.60 0.45 Subcritical 0.5 7.07 2.36 0.33 0.07 5.18 2.20 0.55 Flow 0.6 7.75 3.10 0.39 0.07 8.58 2.77 0.63 0.7 8.37 3.90 0.46 0.06 12.95 3.32 0.70 0.8 8.94 4.77 0.52 0.06 18.36 3.85 0.76 100 Year 0.9 9.49 5.69 0.59 0.05 24.84 4.36 0.81 Storm Level 1 10.00 6.67 0.65 0.05 32.43 4.86 0.86 1.1 10.49 7.69 0.71 0.05 41.15 5.35 0.90 1.2 10.95 8.76 0.78 0.05 51.05 5.83 0.94 1.3 11.40 9.88 0.84 0.05 62.14 6.29 0.97 1.4 11.83 11.04 0.90 0.05 74.44 6.74 1.00 Supercritical 1.5 12.25 12.25 0.96 0.05 87.98 7.18 1.03 Flow

TR-55 Storage Volume Requirements 110100

Drainage Area, Am 0.039 sq.mi. Precipitation 2.70 in. 5.20 in. 7.40 in. Runoff Curve Number, CN = 65

Peak inflow discharge, qi 31.00 cfs 16.00 cfs 31.00 cfs

Peak outflow discharge, qo 20.00 cfs 12.00 cfs 20.00 cfs

compute qo/qi 0.645 0.750 0.645

solve for vs/vr 0.226 0.193 0.193 Runoff, Q…… in 0.38 in. 1.79 in. 3.41 in.

Runoff Volume, Vr 0.78 ac-ft 3.72 ac-ft 7.11 ac-ft

Storage Volume, Vs 0.18 ac-ft 0.72 ac-ft 1.37 ac-ft Storage Volume 7,714 cu. ft. 31,284 cu. ft. 59,750 cu. ft.

Page 33 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Storage Capacity within one Pool Incremental Cumulative Stage Surface Area Storage (ft3) Storage (ft3) 0 0.00 0.00 0.00 0.1 0.48 0.02 0.02 0.2 1.92 0.12 0.14 0.3 4.32 0.31 0.46 0.4 7.68 0.60 1.06 0.5 12.00 0.98 2.04 0.6 17.28 1.46 3.50 0.7 23.51 2.04 5.54 0.8 30.71 2.71 8.25 0.9 38.87 3.48 11.73 1 47.99 4.34 16.08 1.1 58.07 5.30 21.38 1.2 69.10 6.36 27.74 1.3 81.10 7.51 35.25 1.4 94.06 8.76 44.00 1.5 107.97 10.10 54.11 1.6 122.85 11.54 65.65 1.7 138.69 13.08 78.72 1.8 155.48 14.71 93.43 1.9 173.24 16.44 109.87 2 191.95 18.26 128.13 2.1 211.63 20.18 148.31 2.2 232.26 22.19 170.50 2.3 253.86 24.31 194.81 2.4 276.41 26.51 221.32 2.5 299.93 28.82 250.14 2.6 324.40 31.22 281.35 2.7 349.83 33.71 315.07 2.8 376.23 36.30 351.37 Selected 2.9 403.58 38.99 390.36 Depth of 3 431.89 41.77 432.13 Pool 3.1 461.16 44.65 476.78 3.2 491.40 47.63 524.41 3.3 522.59 50.70 575.11 3.4 554.74 53.87 628.98 3.5 587.85 57.13 686.11 3.6 621.92 60.49 746.60 3.7 656.96 63.94 810.54 3.8 692.95 67.50 878.04 3.9 729.90 71.14 949.18 4 767.81 74.89 1024.06 Storage provided in pools = 432 x # of pools (17) = 7,436 ft3

Page 34 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Abbreviated List of Native Plants

Step Pool Storm Conveyances are designed with the intention that they will mimic natural processes. Vegetation plays an important role in these processes. It is highly encouraged on all projects and required on those in Anne Arundel County to use native vegetation appropriate for the conditions of the site.

The following is a sample, abbreviated list of native plants that may be used on SPSC structures within the airport zone. The list has been cross-checked for consistency with the Maryland Aviation Administration (MAA) approved plant list. This list maybe subject to expansion to accommodate other native plant species and future updates to the MAA guidelines. It is the responsibility of the designer to check and propose native plant species that are consistent with MAA regulations for projects within the airport zone.

Common Name Latin Name Comments American Holly Ilex opaca (Male Only) Bald Cypress Taxodium distichum Bayberry Myrica pensylvanica Blue Flag Iris Iris versicolor Christmas Ferns Polystichum acrostichoides Cinnamon Fern Osmunda cinnamomea Fringe Tree Chionanthus virginiana (Male Only) Inkberry Ilex glabra (Male Only) Little Bluestem Schizachyrium scoparium Mountain Laurel Kalmia latifolia Pitch Pine Pinus rigida Switchgrass Panicum virgatum Summersweet Clethra alinifolia Sweetbay Magnolia Magnolia virginiana Tussock Sedge Carex stricta Virginia Sweetspire Itea virginica

For SPSC projects outside of the airport zone, the designer shall utilize the list of native plants as listed below:

Common Name Latin Name American Holly Ilex opaca Atlantic White Cedar Chamaecyparis thyoides Bald Cypress Taxodium distichum Bayberry Myrica pensylvanica Blue Flag Iris Iris versicolor Broomsedge Andropogon virginicus Christmas Ferns Polystichum acrostichoides Cinnamon Fern Osmunda cinnamomea Common Winterberry Ilex laevigata

Page 35 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010 Design Guidelines for Step Pool Storm Conveyance Anne Arundel County Government Department of Public Works, Bureau of Engineering

Common Name Latin Name Cranberry Vaccinum macrocarpon Early Lowbush Blueberry Vaccimiun pallidum Fragrant Water Lily Nymphea odorata Fringe Tree Chionanthus virginiana Golden Club Orontium aquaticum Highbush Blueberry Vaccinum corybosum Inkberry Ilex glabra Little Bluestem Schizachyrium scoparium Mountain Laurel Kalmia latifolia Pitch Pine Pinus rigida Redhead Grass Potamogeton perfoliatus Red Cedar Juniperus virginiana Royal Fern Osmunda regalis Serviceberry Amelanchier canadensis Switchgrass Panicum virgatum Smooth Alder Allnus serrulata Sour Gum Nessa sylvatica Summersweet Clethra alinifolia Swamp Azalea Rhododendron viscosum Swamp Bayberry Myrica heterphylla Sweetbay Magnolia Magnolia virginiana Tussock Sedge Carex stricta Virginia Chain Fern Woodwardia virginica Virginia Sweetspire Itea virginica Wax Myrtle Myrica cerifera Winterberry Ilex laevigata Yellow Pond Lilly Nuphar advena

A complete list of native plants can be found under www.aacounty.org/IP/Resources/AANativePlants.pdf. Special attention shall be placed on diversity and dense placement of plant material within appropriate wetness zones throughout the site (MDE, 2000). Additional information on native plants for the Chesapeake Bay region can be found at www.nps.gov/plants/pubs/chesapeake. For information concerning Native Plant Nurseries, please visit www.aacounty.org/IP/Forms.cfm and scroll down to the "Forestry Forms and Fact Sheets" section.

Page 36 of 36 Step Pool Storm Conveyance (SPSC) Guidelines – Revision 2: November 2010